Inks used in the ink-jet printing industry are typically liquid dispersions, solutions, or emulsions. Known types of ink include oil-based inks, non-aqueous solvent based inks, water-based inks, and solid inks. The ink-jet printing process involves jetting droplets of ink from orifices of a print head onto a print medium. Then, the deposited ink droplets are either affirmatively dried, e.g., using heat or forced air, or allowed to dry at ambient conditions.
Recently, curing of ink by radiation, and in particular ultraviolet (UV) curing has become somewhat popular. In these cases, special radiation-curable ink is used and the image is cured by exposure to a radiation source. The term “curing” in the context of the present application refers to a process of converting a liquid, such as ink, into a solid by exposure to actinic radiation. The use of radiation-curable inks and the curing process are rapidly becoming an alternative to the established conventional drying process.
Among the problems associated with the use of UV curable inks is poor adhesion to non-porous or low surface tension substrates such as polypropylene, vinyl, polyolefins, and the like. Additionally, depending on the ink load, when printing UV curable inks on top of one another, proper cohesion between ink layers on substrate can be less than desirable. Further, often UV curable inks are prepared using oligomers such that the inks have a viscosity that is much higher than more typical solvent or water based inks. Jetting of such viscous inks presents certain difficulties, and further, can only be jetted at low frequencies, causing slower printing speeds. Because these inks are more viscous, they are also usually jetted at elevated temperatures, e.g., above about 40° C. This elevated temperature allows some control over the inks otherwise high viscosity, but adversely affects the life of printing architecture including the print head.
Another problem associated with the use certain UV curable inks is their relatively high gas (air) content that forces use of so called “lungs” or gas extraction devices in the printer ink system or in the print head itself especially while printing at very high frequencies. This causes increase in the cost of the printers and frequent print head purging and maintenance cycles. Another downside of UV curable inks is that these inks are usually accompanied by emission of unpleasant odors. These odors can disturb the user and often require special air trapping and evacuation systems. Thus, there is significant room for improvement in the area of UV curable inks.